1. Field of the Invention
The present invention relates to a process for producing a porous product having pores formed by anodization.
2. Description of the Related Art
For formation of a fine pore structure on a surface of an product, a known technique is an anodization process in which aluminum, silicon, or a like material is immersed as an anode in a solution, and is etched by application of an electric field to form fine pores of nanometers.
For example, an aluminum substrate is immersed in an acidic electrolyte solution such as an oxalic acid solution and a phosphoric acid solution, and an electric field is applied thereto to form a porous anodized film on the surface of the aluminum (R. C. Furneaux; W. R. Rigby; A. P. Davidson: NATURE, Vol.337, p147 (1989)). This porous film is characterized by a special structure having fine cylindrical pores of several to several hundreds of nanometers in diameter (alumina nanoholes) arranged at intervals of tens to hundreds of nanometers. The fine pores have a very high aspect ratio and are uniform in the diameter of the pore cross-section.
The intervals and aspect ratios of the pores of the produced pore structure can be controlled by the anodization conditions. Generally, the pore intervals are proportional to the applied voltage in the anodization; the pore depth is proportional to the anodization time length; and the pore diameter can be enlarged after the anodization by immersion of the alumina in an etching solution capable of uniform etching such as an aqueous phosphoric acid solution.
The nanoholes produced thus in alumina are random in the arrangement, and are irregular at and near the surface although they are perpendicular inside the aluminum film. To improve the irregularity, a two-step oxidation process is known (Japanese Journal of Applied Physics, Vol.35, part 2, No.1B, pp125-129 (1996)). In this process, the anodization is interrupted and the formed porous alumina film portion is removed, and then the anodization of the remaining alumina is continued further to form pores with high perpendicularity, straightness, and independency. After the removal of the porous film formed in the first anodization, pits are left on the surface of the alumina at the positions of the removed nanohole bottoms. These pits serve as the starting points of the pore formation in the second anodization. Therefore, the pore formation is started at the same time without disorder of perpenducularity of the pores at or near the film surface.
In another method, the pores are formed regularly spontaneously by etching for a long time under a certain anodization conditions called a self-regularization conditions (bath composition, bath temperature, applied voltage, etc.) (Journal of Applied Physics, p6023 (1998)).
However, in this method, for achieving a certain regularity, the aluminum film used should be sufficiently thick and be anodized for a long time.
The two-step anodization process as described above is excellent in that the pores can be formed by direct etching only with an intended pore interval and a high aspect ratio without conducting steps of resist application, light exposure, development, and the like, being different from conventional photolithography. Otherwise, a regular pore structure having uniform pore diameters can be formed in somewhat separated domains under self-regularization conditions.
However, in the two-step process, the anodization is interrupted once and the porous film having formed is removed. In this process, the final porous film thickness is not readily controllable.